In general, a multi-stage gear shift mechanism of an automatic transmission is implemented by combining a plurality of planetary gear sets. A powertrain having the combined planetary gear sets serves to vary torque input from a torque converter and transfer the varied torque to an output shaft.
The more the automatic transmission has implementable gear shift stages, the better a gear ratio may be properly designed and power performance and fuel efficiency may be enhanced. Therefore, a research for implementation of more gear shift stages has been continuously carried out.
Durability, power transfer efficiency, size, and the like of the powertrain are varied according to combination of the planetary gear sets, even though the same gear shift stage is implemented. Therefore, a research for a compact powertrain having high strength and less power loss is ongoing.
Particularly, for a manual transmission, many gear shift stages cause a driver the inconvenience of frequently changing speed. However, for an automatic transmission, a control unit performs gear shift by controlling the operation of the power train according to a driving state. Therefore, a research for the powertrain capable of implementing more gear shift stages has been continuously carried out.
In recent years, a six-speed automatic transmission is developed to be applied to the vehicle, and an eight-speed or more automatic transmission has been actively developed to realize higher fuel efficiency.
The eight-speed or more automatic transmission is mainly developed such that three planetary gear sets or four planetary gear sets are combined with a plurality of clutches and brakes. However, it is difficult to secure a space for arrangement of the clutches since all of the clutches are intensively provided at a rear end of the transmission. Accordingly, there are problems in that the whole length of the transmission is increased according to the intensive arrangement of the clutches and passages for implementation of the clutches are complicatedly designed since the clutches are intensively provided at the rear end of the transmission.
Meanwhile, the eight-speed or more automatic transmission is configured such that an output shaft is typically provided to the rear end of the transmission. This configuration in which the output shaft is provided to the rear end of the transmission has an adverse effect on vibration and noise characteristics of the transmission itself, and particularly, is undesirable for a front-wheel-drive vehicle.